Electrophotographic photoreceptors are generally formed by providing a light-sensitive layer on an electrically conductive substrate. Light sensitive layers are commonly made of materials having photoconductivity such as inorganic photoconductive materials (e.g. Se, CdS and ZnO) and organic photoconductive materials. Amorphous silicon and carbon have recently been proposed for use as photoconductive materials (see, for example, JP-A-54-86341 (the term "JP-A" as used hereinafter means an "unexamined published Japanese patent application"). Electrophotographic photoreceptors having light-sensitive materials made of amorphous silicon are principally formed by glow discharge. The resulting photoreceptors have the advantage of high sensitivity. Amorphous carbon has a very hard surface and withstands many cycles of use. In addition, amorphous carbon is not liable to change in quality. Hence, photoreceptors using light-sensitive layers made of amorphous carbon have the advantage of long service life.
However, electrophotographic photoreceptors using the materials listed above do not possess all of the characteristics that are required of photoreceptors to be used in electrophotography, and in commercial applications optimum conditions have to be searched for in accordance with the specific object of use. For instance, two major characteristics that are required to be possessed by electrophotographic photoreceptors are high sensitivity and high dark resistance. However, highly sensitive photoreceptors generally have small dark resistance and they often exhibit fatigue in their properties. Taking a photoreceptor having a Se-based photoconductive layer, for example, since selenium used alone has a narrow range of spectral sensitivity, sensitization is effected by addition of Te or As. Further, a single-layered structure containing Se is seldom used and a more common layer arrangement is a double-layered structure consisting of a Se layer and a SeTe layer, or a three-layered structure consisting of a Se layer, a SeTe layer and a Se layer. On the other hand, Se-based photoconductive layers containing Te or As suffer increased light fatigue, which causes a decrease in image density to either produce a ghost or deteriorate image quality.
Another fundamental characteristic that is required of electrophotographic photoreceptors is longevity of their life but photoreceptors using Se-based photoconductive layers do not have satisfactorily long life. For instance, Se in these photoreceptors is used in the amorphous state but it starts to crystallize at fairly low temperatures of 50.degree. to 60.degree. C. If crystallization occurs, the dark resistance of the photoreceptor decreases to cause deterioration of copied image.
The recently proposed photoreceptors using amorphous silicon as a photoconductive material have the advantages of high sensitivity, high resistance to cyclic use and long service life. However, because of high dielectric constant, a large charging current must be applied or the process speed must be increased in order to attain a desired surface potential. The application of a large charging current results in increased powder consumption and several problems must be solved before the system can be used at a higher process speed. The photoreceptors using amorphous silicon as a photoconductive material have the additional disadvantage that their resistance will vary greatly on account of external factors such as temperature and humidity to influence on charged potential, particularly in a hot and humid atmosphere. Further if a thin film made of an insulating material such as SiO.sub.2 or SiN is formed on the surface of these photoreceptors as a barrier layer to prevent injection of charges, electric conductivity in a direction parallel to the interface will increase to cause occasional production of a blurred image. Further, the photoreceptors using amorphous silicon as a photoconductive material is so structure-sensitive that in order to insure good reproducibility of film formation, the conditions of fabrication and the amount of impurities to be added must be strictly controlled.
Electrophotographic printers that perform scanning with a laser beam on lines have conventionally used gas lasers that operate at comparatively short wavelengths, such as a He-Cd laser, an Ar laser and a He-Ne laser, but the use of semiconductor lasers as the source of laser beams has increased these days. Semiconductor lasers usually emit in the wavelength range longer than 750 nm and various proposals have been made to design electrophotographic photoreceptors that have a high-sensitivity characteristic in such a long wavelength range. For instance, it has been proposed that sensitization for longer wavelengths be effected by incorporating Ge into photoreceptors including those which use amorphous silicon as a photoconductive material (see JP-A-54-98588 and JP-A-57-172344). However, if photoreceptors having a high-sensitivity characteristic in the long wavelength range is exposed to light from a light source emitting at long wavelengths, particularly to a scanning semiconductor laser beam on an electrophotographic printer, moires will be produced to preclude the formation of an image of good quality.
As described above, the conventional electrophotographic photoreceptors in common use have their own merits and demerits and in commercial applications, optimum conditions have had to be searched for in accordance with the specific object of use.